Transmission of electrical and optical signals may be effectively used to communicate information. Microwave signals are electromagnetic emissions transmitted through the air to carry audio and visual information over distance. Similarly, electrical signals may be transmitted over cables, as is done in telephone systems and cable television systems.
Recently, in communications systems, such as telephone systems, emphasis has turned toward use of optical signals and away from use of electrical signals. In modern times, telephone companies implemented optical fiber communication systems. In the home electronics industry phonographs have been replaced with compact disk players, which rely upon laser light reflections to read information from a disk.
Practical reasons exist for the shift in focus to optically driven systems. Unlike electrical signals, optical signals are generally unaffected by electromagnetic fields created by such things as power lines, lightning and even sunspots. These sources of interference may create noise in electrical signals. Noise may appear, for instance, as static in an audio signal or distortion in a visual signal. Thus, while such electromagnetic fields create noise in an electrical communication system, an optical system retains its original qualities in the presence of the electromagnetic fields.
Information capacity of optical signals is also much larger than lower frequency electrical signals that are used in wire and wireless communication systems. Generally, higher frequency signal carriers provide larger information capacity than lower frequency signal carriers. This is due to the wider bandwidth of the higher frequency signals.
Larger information capacity and noise immunity are great benefits, but another important benefit of communicating with optical signals is the small size of optical fibers used as a transmission medium. A typical fiber having hair sized dimensions is a suitable replacement for bundles of copper wires having much larger diameter. As demands for information access become larger and larger in modern times, the use of optical transmission systems places less demand on space in the construction of underground, above ground, and internal building communication systems.
Common difficulties are encountered in the practical implementation of optical communication systems, however. Ideally, the basic elements of a communication system include a transmitter, a transmission medium, and a receiver. Input signals, typically electric signals, are input to an optical transmitter. Conversion of the input signal to an optical signal is conducted within the transmitter and a light source, such as a semiconductor laser, pumps light into the optical transmission medium. The transmission medium usually takes the form of an optical fiber. Reception and conversion of the optical signal is accomplished in a receiver coupled to the optical fiber at some distance away from the transmitter. A basic receiver will include a light detector for detecting the optical signal and converting the same to an electrical signal, an amplifier for amplifying the electrical signal, and signal reproducer for outputting the original input signal as an electrical signal.
In practice, additional elements are required since signal losses occur over distance in the optical fiber. Losses limit the distance by which the transmitter and receiver may be separated. These losses are generally referred to as optical signal attenuation. Absorption of signal light by the fiber acting as the transmission medium is one factor causing attenuation. Other factors leading to attenuation are the scattering of the signal light over a wider wavelength than the original transmission and radiative losses, typically occurring at bends in the optical fiber. Combination of these individual losses leads to a total signal attenuation characteristic for a particular optical transmission medium which is measured in decibels per kilometer.
In order to implement practical systems, taking into consideration the optical attenuation characteristic of the particular optical fiber being used, it is therefore necessary to periodically amplify the signal as it travels over distance. Repeater stations are used to accomplish this amplification and are an integral part of modern optical communication systems. Typical repeater stations include both a receiver and transmitter which decode the optical signal, convert it to an electrical signal, reconvert to an optical signal and transmit the optical signal toward the next repeater or receiver station.
Repeater stations contribute significantly to the cost of optical communication systems, commonly costing tens of thousands of dollars. Moreover, repeater stations are provided in redundant pairs or larger numbers of repeaters, since a repeater may fail. Additionally, the repeater stations are often installed in inconvenient locations, such as the ocean floor, that makes replacement and initial installation difficult and expensive.
A simpler manner of implementing repeater stations involves use of optical amplifiers. The general structure of an optical amplifier is detailed in U.S. Pat. No. 5,309,452 to Ohishi et al., which is hereby incorporated by reference. In an optical amplifier, the signal light is amplified in optical form without conversion to an electrical signal. Amplification is accomplished by the signal stimulating additional emission as it passes through the optical amplifier. Of course, the optical amplifier has other applications, including implementation at the transmission end of an optical communication system to create stronger optical signals that may travel further in a fiber having given attenuation characteristics.
Optical amplification is attributable to what is referred to as stimulated emission. A rare-earth-doped glass (usually optical fiber) is pumped at a wavelength which is absorbed by the rare earth ions placing them in an excited state. The rare earths can relax to the ground state by emitting characteristic longer wavelength photons (compared to the pump light) in a process called spontaneous emission. If the pumping intensity is sufficiently high, the rate of excitation can exceed the spontaneous decay rate and a population inversion is obtained. When pulses of signal light having photon wavelength within the spontaneous emission band of the inverted population of excited rare earth ions enter the amplifier, they stimulate the emission of additional photons of identical wavelength, phase and propagation direction (stimulated emission). These additional photons imparted to the signal are responsible for the gain of the optical amplifier.
Traditionally, the optical amplifiers and transmission media have been formed with oxide glasses. A widely applied amplifier using oxide glass is the Erbium doped fiber amplifier (EDFA). More recently, chalcogenide glasses have been investigated as hosts since these glasses have good infrared wavelength transparency, are durable, are easy to prepare in bulk or thin film form, can form optical fibers, and may be formed as patterned waveguides by photodarkening processes. The ability to create chalcogenide thin films, by sputtering, for instance, allows for formation of a device using a chalcogenide glass as part of a larger semi-conductor integrated package.
Typical EDFA's rely exclusively upon the pumping absorption and emission characteristics attributable to the dopant, i.e. Erbium (Er). Effective absorption of light from the excitation source by the EDFA requires that the excitation light correspond to narrow characteristic absorption bands of the Erbium dopant. Incident light in these bands will excite electrons of Erbium ions within the glass to higher energy levels, and photons are released to provide luminescence when the electrons return to the normal state. Typical amplifiers rely upon and are limited to use of the luminescence bands attributable to the radiative transitions of the dopant. Co-doped amplifiers have also been used incorporating Er with Yb, wherein Yb enhances the absorption and subsequently transfers its energy to the emitting Er. Pr is an additional element used as a dopant.
Reliance upon dopant radiative transition emission characteristics for light output results in pumping systems having narrow output emission bands. In the art, the commonly employed emission bands are the 1340 nm Pr band and the 1550 nm Er band. These bands are fairly narrow. For instance, a typical EDFA has an approximately 30 nm wide 1550 nm emission band. Similar result is reached by employing the 1340 nm Pr band.
In the case where an EDFA is used as part of a communication system, the narrow band limits information capacity. A fiber link may carry a number of distinct signals operating at different wavelengths. However, a given separation on the wavelength spectrum is necessary to avoid "cross-talk" between the signals resulting from attenuation as well as the ability of a receiver to distinguish among signals at closely spaced wavelengths. Expansion of the emission band directly relates to expansion of the number of optical channels which may be supported.
Another difficulty associated with narrow bandwidth relates to multi-stage pumping systems. In such a system, a number of amplifiers, such as EDFA's may be cascaded. For the system to be useful, the gain profile over the bandwidth of the amplifiers must be uniform. While such uniformity is important in a single stage amplifier, it becomes critical in a cascaded arrangement since any non-uniformity in gain across bandwidth will be magnified with each successive stage of amplification.
Other applications utilizing pumping systems also encounter difficulties associated with narrow and non-uniform emission bands. Generally, the problems associated with communication applications of relatively narrow band pumping systems also apply to other devices, such as lasers. Pumping relying upon a narrow rare earth dopant absorption band is limited by the weak oscillator strength of the rare earth absorption. In addition, a pumping source, such as a laser, of a specific wavelength is required. Additional flexibility should be provided in the implementation of a pumping source through use of the pumping system in related application no. 08/435,353 (CHALCOGENIDE OPTICAL PUMPING SYSTEM DRIVEN BY BROAD ABSORPTION BAND, Bishop et al.), which uses a continuous broad absorption band of approximately 400 nm in width.
In sum, there is a need for an improved optical pumping system having a broad and continuous photoluminescence emission band providing great flexibility in the application of the pumping system to optical amplifiers, communication systems, lasers and other devices. Good oscillator strength should be provided, and the pumping system should allow for controlled gain across the broad emission band.
It is therefore an object of the present invention to provide an improved optical pumping system that uses a photoluminescence emission band extending beyond characteristic dopant emission transitions.
An additional object of the present invention is to provide an improved optical pumping system utilizing a continuous optical emissions band of approximately 190 nm in width.
Another object of the present invention is to provide an improved optical pumping system responsive to excitation pumping light over a continuous broad absorption range of approximately 400 nm and which produces optical emission over a continuous emission band of approximately 190 nm in width.
Still another object of the present invention is to provide an improved optical pumping system which can provide photoluminescence emissions over a continuous emission band of approximately 190 nm in width and utilize excitation sources, such as light emitting diodes and simple semiconductor lasers, that supply pumping light over a broader spectrum than finely tuned and expensive laser sources.
An additional object of the invention is to provide an improved optical pumping system having a chalcogenide glass doped with a rare earth, and which absorbs excitation light from an excitation source over an approximate wavelength band of 600-1064 nm and produces photoluminescence emissions in response thereto over an approximate wavelength band of 1510 nm-1700 nm.
A still further object of the present invention is to provide an improved optical pumping system having a chalcogenide glass doped with a rare earth responsive to an excitation source, the glass having a broad absorption band and responsive emission band, and being suitable for thin film deposition in an integrated circuit including the excitation source.
Yet another object of the present invention is to provide an improved optical pumping system having a chalcogenide glass co-doped with Erbium and Praseodymium, the chalcogenide glass absorbing pumping light from an excitation source over a broad absorption band of approximately 400 nm in width, and producing photoluminescence emissions in response thereto over an approximate wavelength band of 1510 nm-1700 nm.
A still additional object of the present invention is to provide an improved optical pumping system having a rare earth doped chalcogenide glass in which electrons are excited to a higher energy level in response to excitation source light over a broad absorption range of approximately 400 nm in width, the electrons having higher energy level occupancy lifetimes in the approximate range of 0.25 ms (Praseodymium) to 2 ms (Erbium), and producing optical emission over a continuous broad range of approximately 190 nm in width.